In a general spindle motor, rotation shaft is forcedly inserted in T-bush, and the T-bush is coupled with clamp plate, turntable and ball case storing balls. In other words, the T-bush, the clamp plate, the turntable and the ball case are fabricated separately. As a result, the clamp plate is coupled with the upper surface of the T-bush. The turntable and the ball case are coupled with an outer cylindrical surface of the T-bush. The T-bush is forcedly inserted into the rotation shaft. So, the conventional spindle motor as described above is fabricated very complicatedly. Fabrication cost also rises due to the complicated fabrication. To overcome the problem as described above, there was proposed a spindle motor consisting of a T-bush, a clamp plate, a turntable and a ball casing.
FIG. 1 is a cross sectional view of a spindle motor having an integral turntable. Referring to FIG. 1, a bearing housing 21 is installed on a plate 10 and a bearing 23 supporting a rotation shaft 30 is forcedly inserted in an inner cylindrical surface of the bearing housing 21. A core 41 is coupled with an outer cylinder of the bearing housing 21. A coil 43 is wound on the core 41. A rotor yoke 51 surrounding the core 41 is installed at the almost center of the rotation shaft 30. A magnet 53 opposed to the core 41 is installed on an inner cylindrical surface of the rotor yoke 51. An integral turntable 60 is coupled with an upper portion of the rotation shaft 30. A disk 70 is mounted on an upper surface of the integral turntable 60. A ball 61 and a ball cover 63 are installed in the integral turntable 60 to reduce vibration caused by eccentricity of a rotating body. Balancing operation of the ball 61 will be described schematically.
When a spindle motor is rotated, a rotating body is vibrated due to the integral turntable 60 as itself and the eccentricity of the disk 70 installed on the turntable 60. Here, when the revolution per second of the motor exceeds the resonance frequency of a set, the ball 61 contacted to the inner cylindrical surface of an outer wall 60a of the turntable 60 moves to the opposite direction of eccentricity and stops at the location at which the eccentricity of the rotating body is compensated owing to centrifugal force caused by rotation of the rotating body and frictional force at a contact surface of the balls so that vibration is reduced.
Since centrifugal force depends on the revolution of the rotating body, vibration reduction of the spindle motor is controlled according to the frictional force of the contacted frictional surface between the balls. In other words, if a frictional force is weak, vibration reduction is excellent and stabilization revolution of a spindle motor is achieved at a high revolution. On the contrary, if a frictional force is strong, vibration reduction is reduced and stabilization revolution of a spindle motor is achieved at a low revolution.
On the other hand, it is not desired that the balls 61 move continuously even in normal operation state of the spindle motor since if a frictional force of the contact surface between the balls 61 is weak, the stabilization revolution gets high. For example, in the case of an audio compact disk, the spindle motor should operate at 3,700 rpm in a normal state. However, if a friction force is weak and a stabilization revolution approaches to or exceeds 3,700 rpm, the spindle motor does not operate stably since the balls fluctuate.
For this problem, the roughness is enhanced in the related art in order to enhance the frictional force. However, in case that a T-bush, a clamp plate, a turntable and a ball contacting surface are integrated as an integral turntable, an inner cylindrical surface of an outer wall 60a as a contacting surface of ball 61 is difficult to form to have a proper roughness in its manufacturing procedure. Even though it has a proper roughness, it is practically difficult to make a satisfactorily reliable roughness in its manufacturing procedure to obtain a proper stable revolution. Even if all products are produced in the same condition, the products are different from each other in their roughness. Accordingly, since their stabilization revolutions are possibly set to be differently, it is very difficult to adjust frictional force by making roughness in manufacturing spindle motors employing integral turntables.